Larger Exit Pupil

[jetstream]

8 minutes ago, Alien 13 said:

It would be interesting to find out the role of a big objective when the exit pupil might be 20 mm or so, my thoughts are that all of the objective is still used for example you could have a big hole in any part or the middle like a Mak and still produce a complete image.

Alan

All of the objective is not used but the complete image will be there albeit without as much "information". Its interesting when I observe the moon though the trees...I still get the image. Also trying weird shape aperture masks will still focus the final image into the shape of the object...

[Alien 13]

There has to be an advantage with large exit pupils most if not all military kit from tank sights, missile trackers, spotters, rifle scopes to periscopes use them.

Alan

[jetstream]

This little chart offers some ideas on exit pupil vs dimming- and as we know extended objects dim equally with the sky using a reduced exit pupil. Another fun thing to read is Feynmans book on QED.

 

[Piero]

1 hour ago, jetstream said:

The question is: what does aperture really do?

Some say it only plays a role in the needed or desired object size at a certain illumination?

Aperture determines the amount of signal. Taken two telescopes of equal focal length, the one having a larger aperture will show a brighter image, because it collects more signal. 

As we know, an exit pupil greater than 7mm tends not to be useful even under dark skies (although there are people who intentionally go beyond this for specific targets). Therefore, to optimise the exit pupil, we play with that signal by shifting the image towards us, increasing the image scale.

So, I'd answer that: 

- the role of aperture is to collect signal

- the role of telescope / eyepiece focal length is to shift the image scale so that we are within 0-7mm exit pupil (which works well for our eyes). On the other hand this range corresponds to a range of magnifications, which corresponds to a range of object image scale.

 

6 minutes ago, Alien 13 said:

It would be interesting to find out the role of a big objective when the exit pupil might be 20 mm or so, my thoughts are that all of the objective is still used for example you could have a big hole in any part or the middle like a Mak and still produce a complete image.

Alan

I presume you mean 2mm, otherwise we need to invite Gollum for such a test!  

[jetstream]

1 minute ago, Alien 13 said:

There has to be an advantage with large exit pupils most if not all military kit from tank sights, spotters, rifle scopes to periscopes use them.

Alan

Could it be that the large exit pupil ocular also give a very large TFOV?

[Alien 13]

1 minute ago, Piero said:

Aperture determines the amount of signal. Taken two telescopes of equal focal length, the one having a larger aperture will show a brighter image, because it collects more signal. 

As we know, an exit pupil greater than 7mm tends not to be useful even under dark skies (although there are people who intentionally go beyond this for specific targets). Therefore, to optimise the exit pupil, we play with that signal by shifting the image towards us, increasing the image scale.

So, I'd answer that: 

- the role of aperture is to collect signal

- the role of telescope / eyepiece focal length is to shift the image scale so that we are within 0-7mm exit pupil (which works well for our eyes). On the other hand this range corresponds to a range of magnifications, which corresponds to a range of object image scale.

 

I presume you mean 2mm, otherwise we need to invite Gollum for such a test!  

I did mean 20 mm and its not uncommon in certain applications.

Alan

[jetstream]

10 minutes ago, Piero said:

the one having a larger aperture will show a brighter image

Many disagree with this Piero, but my mind is open to more thoughts on the actual effect of aperture.

Edit: I re read you post Piero, I took this quote out of context- my apologies!

[Alien 13]

2 minutes ago, jetstream said:

Could it be that the large exit pupil ocular also give a very large TFOV?

I dont know but from experience you can stick your eye almost anywhere and see the image, I have had a few combinations with some scopes of having to hover around to even see a glimpse of light and it would vanish a few mm in either direction.

Alan

[Stu]

7 minutes ago, jetstream said:

Many disagree with this Piero, but my mind is open to more thoughts on the actual effect of aperture.

Is this not the case of aperture not being able to increase surface brightness, but it can increase what you might term overall brightness i.e. Surface brightness x area?

[Piero]

14 minutes ago, jetstream said:

Many disagree with this Piero, but my mind is open to more thoughts on the actual effect of aperture.

are you talking about the eye or in general? 

I'm talking generally. If f is the focal length for two telescopes of different aperture A' and A'', e.g. A' < A'', then A'' collects more signal. If we attach two cameras to these two telescopes, A" will require a shorter exposure (shorter focal ratio) for capturing an image with comparable brightness. If you apply the same exposure, again A" will show more signal. 

 

Edit:

Of course if this signal cannot be processed successfully, it is useless.

[Piero]

4 minutes ago, Stu said:

Is this not the case of aperture not being able to increase surface brightness, but it can increase what you might term overall brightness i.e. Surface brightness x area?

Do you mean noise?

[Alien 13]

4 minutes ago, Stu said:

Is this not the case of aperture not being able to increase surface brightness, but it can increase what you might term overall brightness i.e. Surface brightness x area?

I think thats probably correct which is exactly what you need for military applications if trying to see in near darkness.

Alan

[jetstream]

2 minutes ago, Stu said:

Is this not the case of aperture not being able to increase surface brightness, but it can increase what you might term overall brightness i.e. Surface brightness x area?

I quoted Piero out of context Stu and am wrong for doing that.

The idea that larger aperture collects more light and therefore offers an advantage for us visually is disputed ie many believe that exit pupil and the objects size in the EP are the only factors to consider when viewing extended objects and if more aperture offers advantages because of its effect on the other 2 then the apertures advantage is recognized.

Personally I believe more aperture gives advantages and not just on extending the MTF range on solar system objects. I have been "corrected" (hammered ) about my belief lol!

There is a "new" idea of light through put in a telescope however and also the concept of "entendue" which I'm trying to understand. I'll say this- my little 200mm f3.8 newt does a VG job on large nebula ( which purists will say is because of the large TFOV and large exit pupil).

I hope the OP can gain some insight into this from these posts... understanding these concepts as best we can will help immensely with our continuing pursuit of better views IMHO.

[Piero]

22 minutes ago, jetstream said:

Many disagree with this Piero, but my mind is open to more thoughts on the actual effect of aperture.

Edit: I re read you post Piero, I took this quote out of context- my apologies!

No problem Gerry!  

Let's carry on with this interesting thread then!  

[Stu]

9 minutes ago, Piero said:

Do you mean noise?

No, not in this instance 

[jetstream]

There might be something to the SN with respect to the entrance/exit pupil and the physiology of the eye/brain. Sometimes in my posts I refer to something that "pleases the eye" etc and I believe there are combinations out there that do, in general and also individually.

I do believe that a larger apparent field enhances the view of certain objects such as large nebulae for instance. Piero correctly (IMO) pointed out to me that an OIII (for example) helps the SN ratio much in our favor on emission nebula.

There is much to learn...

[cloudsweeper]

2 hours ago, Stu said:

Is this not the case of aperture not being able to increase surface brightness, but it can increase what you might term overall brightness i.e. Surface brightness x area?

This is something that causes argument, Stu.  Some say that a larger image at a given surface brightness merely seems to be brighter, while others assert that constant SB integrated over a larger area (bigger image) results in greater actual  brightness, and that this is expected because the larger aperture takes in more light flux.  I shan't say where I stand on this!

Concerning excessive exit pupil - as I understand (or not!) it, this constitutes wastage of light (some misses the eye), but the larger TFOV still holds, so wide views are still possible (of brighter objects anyway), and "hopping" is still rendered easier.

Doug.

[YKSE]

1 hour ago, cloudsweeper said:

Concerning excessive exit pupil - as I understand (or not!) it, this constitutes wastage of light (some misses the eye), but the larger TFOV still holds, so wide views are still possible (of brighter objects anyway), and "hopping" is still rendered easier.

Doug.

The larger TFOV might indeed contribute to easier redering, I'm not sure about "waste of light i" part. I'd rather to think that it's smaller image scale when using excessive exit pupil, here's a example, an observer with max pupil opening at 5mm, with a 200mm f5 scope, using a 25mm EP, he gets max brightness for his dark adpated eye, the image is magnified in 200*5/25=40 times; using a 40mm EP, his pupil still only gets the same  brightness of 5mm pupil opening, but the image scale is now 200*5/40=25 times. If the DSO is well-framed in both EPs, 40x mag should most likely renderred easier, if only the 25x framed the DSO, then the excessive exit pupil EP might work better.

Check out the discussions (Ernest's answers specifically) on the other site:

https://www.cloudynights.com/topic/564309-aperture-stop-down-caused-by-ep-of-too-large-exit-pupil/

[Special K]

I'm of the opinion (and not being terribly scientific here) that greater aperture is bringing more photons from the source to a point of focus. A light bucket is harvesting more parallel rays of light from a given source and sending that up the tube than a smaller objective can.  I see this dramatic difference between my two refractors and this becomes evident when viewing at the same magnification.  

Another function of aperture is the influence this has in increasing the diffraction limit and therefore increasing the level of detail which can be viewed, such as splitting tighter doubles. The airy disc is smaller in larger aperture scopes. 

This site is fantastic if you like the science and math:

http://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

[cloudsweeper]

6 hours ago, YKSE said:

The larger TFOV might indeed contribute to easier redering, I'm not sure about "waste of light i" part. I'd rather to think that it's smaller image scale when using excessive exit pupil, here's a example, an observer with max pupil opening at 5mm, with a 200mm f5 scope, using a 25mm EP, he gets max brightness for his dark adpated eye, the image is magnified in 200*5/25=40 times; using a 40mm EP, his pupil still only gets the same  brightness of 5mm pupil opening, but the image scale is now 200*5/40=25 times. If the DSO is well-framed in both EPs, 40x mag should most likely renderred easier, if only the 25x framed the DSO, then the excessive exit pupil EP might work better.

Check out the discussions (Ernest's answers specifically) on the other site:

https://www.cloudynights.com/topic/564309-aperture-stop-down-caused-by-ep-of-too-large-exit-pupil/

Yes, for the same (i.e. maximum) exit pupil, the surface brightness of an extended object is the same, but the image is smaller in your second case.  But would not the integrated brightness over a larger image still be greater?  Or if not, then at least the larger image might be perceived to be brighter.

Looking at it another way, if the exit pupil is "maxed", then the f no. is in effect increased (exit pupil = EP focal length / f no.), and this in turn means that the effective aperture is decreased. 

If we consider point sources rather than extended objects, their brightness depends on aperture only, so if excessive exit pupil results in reduction of effective aperture, then stars will appear fainter.

Doug.

 

 

[Stu]

9 hours ago, cloudsweeper said:

I shan't say where I stand on this!

Please do Doug, your secret is safe with me  

Seriously, we all have an opinion and it would be good to hear what you think.  

[cloudsweeper]

27 minutes ago, Stu said:

Please do Doug, your secret is safe with me  

Seriously, we all have an opinion and it would be good to hear what you think.  

Tin helmet on, Stu - I subscribe to the greater integrated  brightness theory, while also believing that a larger image of the same SB would anyway be perceived  to be brighter.  

All good stuff though, don't you think?

Doug.

[YKSE]

54 minutes ago, cloudsweeper said:

Yes, for the same (i.e. maximum) exit pupil, the surface brightness of an extended object is the same, but the image is smaller in your second case.  But would not the integrated brightness over a larger image still be greater?  Or if not, then at least the larger image might be perceived to be brighter.

Looking at it another way, if the exit pupil is "maxed", then the f no. is in effect increased (exit pupil = EP focal length / f no.), and this in turn means that the effective aperture is decreased. 

If we consider point sources rather than extended objects, their brightness depends on aperture only, so if excessive exit pupil results in reduction of effective aperture, then stars will appear fainter.

Doug.

 

 

Yes, I agree with your thoughts about point source objects, that why I usually address extended and point source objects seperately, not using the single word DSO, simply because there are quite some difference the two types.

[Stu]

24 minutes ago, cloudsweeper said:

Tin helmet on, Stu - I subscribe to the greater integrated  brightness theory, while also believing that a larger image of the same SB would anyway be perceived  to be brighter.  

All good stuff though, don't you think?

Doug.

Thanks for posting that Doug, but how could you possibly think such a thing?????

 

 Joking of course. Actually I'm with you, I think exactly the same. If surface brightness remains the same but the area increases then the total brightness must be higher surely?

I'm not sure what units are used to describe the total integrated brightness though?

Anyway, happy to put my hat on and join you under the table  

[John]

Don Pensack has recently posted this on exit pupils on the Cloudynights forum which I think is an interesting contribution to this topic:

https://www.cloudynights.com/topic/592949-no-exit-pupil-limitation-for-refractors/?p=8122944